1. Field of the Invention
The present invention pertains to a synthesis of and a process for making block copolymers of styrene and an unsaturated cyclic anhydride, such as maleic anhydride or itaconic anhydride, via free radical polymerization, in the presence of a stable free radical, a composition of matter comprising block copolymers of styrene and an unsaturated cyclic anhydride, and use of the composition of matter as a compatibilizer in blending polymers.
2. Description of the Related Art
Random copolymers of styrene and an unsaturated cyclic anhydride (UCA), in particular maleic anhydride (MA), with different compositions, have been produced by a number of free radical polymerization processes. One of the end-uses of these materials is the compatibilization of blends of styrenic polymers with other thermoplastics. The content of maleic anhydride in the copolymer, and the copolymer molecular weight, play an important role in the ability of these materials to act as effective compatibilizers. Compatibilizer materials that present a block copolymer structure, in which each one of the blocks is thermodynamically compatible with one of two polymeric materials to be blended, perform more effectively as compatibilizers than their random copolymer counterparts. This permits the use of a smaller amount of compatibilizer material to obtain the desired end-properties in the polymer blend and in some cases may be the only way to achieve the compatibilization of the two otherwise incompatible polymers.
Traditional free radical processes cannot produce block copolymers due to the fact that each individual polymer chain formed has an extremely short life-time (time that it remains as a polymeric free radical). During this short active life it is practically impossible to change the environment surrounding the active chain, so it is not feasible to change the monomer to a second one which would result in a block copolymer. Living polymerization processes, in which termination reactions are suppressed or significantly reduced, allow for the formation of block copolymers, as the life of each individual chain is extended to periods comparable to the duration of the process (minutes or hours). It is possible to produce block copolymers by anionic polymerization, but this technique presents severe limitations for its broad practical application. On one hand, it requires conditions of extreme purity in the monomers because humidity traces destroy the catalyst, and for many monomers it is very difficult to control, requiring extremely low temperatures. Also, the polymerization of monomers having functional groups is not practical since the catalyst can be destroyed by the presence of a number of functional groups. As a result, the industrial application of this technique is reduced to a few monomers, leaving out technologically important functional monomers.
Due to limitations in the anionic polymerization process a more promising technique for producing block copolymers with a large variety of monomers is that based on living or quasi-living free radical polymerization. This can be achieved by adding to an otherwise standard free radical polymerization recipe, a chemical agent that significantly reduces the extent of irreversible termination or chain transfer reactions, conferring a living or quasi-living character to the polymerization, which is also called “controlled polymerization” or “controlled free radical polymerization.” There are several ways to obtain this behavior, but most of them are limited in an industrial practice because they require chemical agents that are not readily commercially available in the market. Among these techniques, one that is particularly useful, and for which the required chemical agents are available in the market, is a quasi-living free radical polymerization controlled by 2,2,6,6 tetramethyl-piperidine-N-oxyl, which is known as TEMPO, and derivatives thereof, which act as stable free radicals capping polymeric growing radicals and uncapping them in a fast and reversible way, allowing for short periods of propagation through monomer-addition steps. U.S. Pat. No. 5,401,804, issued to Georges et al., which is incorporated by reference, describes a polymerization process for producing low polydispersity polymers and block copolymers via a free radical polymerization process that uses a free radical initiator and TEMPO derivatives. However, in order to produce block copolymers, Georges et al. require a sequential addition of monomers, in some cases exhausting the first monomer before adding the second one, resulting in a process with several reaction steps and long overall reaction times.
U.S. Pat. No. 6,531,547 B1, issued to Visger and Lange, discloses a polymerization procedure in the presence of a stable free radical, for the preparation of a block copolymer formed of a vinyl aromatic monomer (which can be styrene) in the first block and a copolymer of a vinyl aromatic monomer and an acrylic monomer (which can be maleic anhydride) in the second block, to be used as an additive for lubricating oil compositions. However, it is believed the process requires the sequential addition of the monomers.
International Patent Publication No. WO 99/47575, issued to Vertommen et al., describes a process for the copolymerization of a vinyl monomer and a maleic monomer in the presence of an initer (e.g. alkoxyamine) for the production of low molecular weight block copolymers. Only the production of low molecular weight polymer is believed to be disclosed in this patent. Additionally, this process requires an alkoxyamine that is not believed to be readily available at the industrial level.
In an improvement on previous techniques, a procedure described in “One-Step Formation of Functionalized Block Copolymers,” Macromolecules, Vol. 33, 1505-1507 (2000) is proposed by Benoit et al. to produce block copolymers containing functional groups via a one-step, quasi-living free radical polymerization mediated by nitroxide compounds. However, in an application to styrene-maleic anhydride copolymers, Benoit et al. were not able to obtain living behavior by only adding a single stable free radical to the standard free radical polymerization recipe. Instead, they had to use a combination of an ?-hydrido-based alkoxyamine and a nitroxide-type stable free radical in order to achieve livingness. This approach is difficult to scale-up in an economic way to an industrial process due to the complexity in synthesizing the alkoxyamine, as described in Benoit et al., Journal of the American Chemical Society, 121, 3904 (1999), as this synthesis involves several reaction steps.
In another attempt of producing block copolymers with functional groups, Park et al., in “Living Radical Copolymerization of Styrene/Maleic Anhydride,” J. Polym. Sci., Part A: Polym. Chem., 38, 2239 (2000), report on the synthesis of diblock copolymers containing a block of styrene-co-maleic anhydride and another block rich in styrene, starting from a mixture of TEMPO, benzoyl peroxide as initiator, and the two monomers. They report some degree of living character in their polymerizations, but they only get polymers having number average molecular weight (Mn) up to 23,500 after 20 hrs. of reaction, which is a chain length generally too low to act as a compatibilizer and also limits its use for other potential applications.
Other attempts tested the chain-end functionalization of polystyrene; that is, the synthesis of styrene polymers having only one functional monomer unit at the end. The general idea of this approach is to use a living or quasi living process of styrene polymerization, which is terminated by adding an excess of a second functional monomer that does not homopolymerize. Harth et al., in “Chain End Functionalization in Nitroxide-Mediated Living Free Radical Polymerization,” Macromolecules, 34, 3856 (2001), report on the synthesis of such materials via quasi-living radical polymerization procedures mediated by alkoxyamines; however, these last compounds are not readily available in the market and their preparation requires several reaction steps. Also, Koulouri et al., in “Terminal Anhydride Functionalized Polystyrene by Atom Transfer Radical Polymerization Used for the Compatibilization of Nylon 6/PS Blends,” employ a similar approach, but using atom transfer radical polymerization (ATRP) to impart living character to the polymerization. This technique, however, suffers from several drawbacks since ATRP requires a metal based catalyst-ligand system, which results in a number of practical problems including metal removal, catalyst removal and/or reuse and color in the polymer. A related way of synthesizing chain end-maleic anhydride functionalized polystyrenes, is to add trimellitic anhydride chloride in order to terminate living chains of polystyrene growing via anionic polymerization, as disclosed by I. Park et al., J. Polym. Sci., Polym. Chem. Ed., 29, 1329 (1991). This creates a single functional group at one end of a polymeric chain. However, this approach suffers from the deficiencies mentioned before and common to all anionic polymerization processes, and it cannot add more than one maleic anhydride unit, limiting the compatibilization versatility of the materials produced.
Yet another attempt of obtaining chain-end functionalized polystyrene is described in U.S. Pat. No. 6,143,848 issued to Lee et al. They perform a controlled radical polymerization via degenerative transfer, using a functionalized iodine reagent. However, a drawback of degenerative transfer is that there is always a low molecular weight radical available for termination, which leads to poor control of the polymerization.
It is well established that the reactivity ratios of styrene and maleic anhydride are nearly zero at temperatures below 80.degree. C., and this results in almost perfectly alternating copolymers. The kinetic data in the literature above 80.degree. C. are scarce, but there seems to be some tendency towards alternation at these higher temperatures. See Zhen Yao et al., Continuous Thermal Bulk Copolymerization of Styrene and Maleic Anhydride, Journal of Applied Polymer Science, 73, 615-622 (1999). The tendency towards alternation of the free radical copolymerization of styrene and itaconic anhydride is less pronounced than in the case of the copolymerization of styrene with maleic anhydride, but it is also present.
With respect to the commercial production of copolymers of styrene and maleic anhydride, special bulk and heterogeneous processes have been developed for the production of random and alternating copolymers. Molar compositions containing less than 10% of maleic anhydride require controlled addition of low amounts of maleic anhydride. Bulk continuous processes oriented to this goal have been described in European Patent No. 27,274, issued Aug. 5, 1984, to Daicel Chemical Industries KK and in Japanese Patent No. 74,313, issued May 10, 1982, to Mitsubishi Monsanto Co., but they result in essentially random copolymers.
Random copolymers of styrene-maleic anhydride (rSMA) have been used as compatibilizers in polymer blends. In a random copolymer the maleic anhydride groups are randomly distributed along the chain of the copolymer. Consequently, the structure of such a compatibilizer cannot be controlled. The key to achieve the desired performance has been the reaction of the maleic anhydride units of the copolymer with a functional group or moiety of one of the polymers included in the blend as well as the miscibility or compatibility of the rSMA with the other components of the blend. However, this in turn has proven to be a shortcoming for the applicability of these copolymers because the miscibility window of rSMA with other polymers is usually narrow, and it is restricted to composition and molecular weight ranges of the copolymer. It is well documented that random copolymers with a maleic anhydride content higher than 8% are not miscible with polystyrene (see Merfeld et. al., Polymer, 39, 1999 (1998), and that its miscibility window with other styrenic copolymers (SMMA, rSMA, SAN) is also restricted (see Gan et. al., J. Appl. Polym. Sci., 54, 317 (1994)). The miscibility of styrene-itaconic anhydride random copolymers shows a similar trend (see Bell et. al., Polymer, 35, 786 (1994)). This limits the applications of rSMA as a compatibilizer for several systems, although it has been sought to compatibilize blends with engineering polymers that contain reactive groups toward the carboxylic functionality of the MA.
Engineering thermoplastics such as polyamides, polyphenylene ethers, polycarbonates and polyesters have excellent physical properties such as strength and stiffness, but it is often required to blend or alloy these with other thermoplastics in order to improve their impact toughness or to reduce their overall cost. However, the components of such blends are usually highly incompatible. It is thus common practice to include a compatibilizer that functions to improve the adhesion between the incompatible components and/or to modify the surface tension at the phase boundaries.
Of particular interest are blends of polyphenylene ethers and polyamides. Such blends are inherently incompatible. Molded articles made from these blends, without a compatibilizing agent, have inferior mechanical properties, such as low impact strength. Numerous attempts to compatibilize this system have been reported, U.S. Pat. No. 4,315,086 describes grafting PPE directly to the polyamide; U.S. Pat. Nos. 4,600,741 and 4,732,937 depict the formation of copolymers of polyphenylene ether and polyamide using an epoxy functionalized polyphenylene ether. U.S. Pat. Nos. 5,231,146 and 5,141,984, and also Chiang et al., in the J. of Appl. Polym. Sc., 61(3), 1996, 2411-2421, portray the use of polyepoxides and compounds containing glycidyl groups to achieve compatibility of the blends. U.S. Pat. No. 6,444,754 discloses the use of an epoxy functionalized oligomer prepared by free radical polymerization of an ethylenically unsaturated monomer or oligomer in the presence of a glycidyl-functionalized nitroxyl initiator.
Other commercially important systems include blends of polycarbonate and polyesters with styrene copolymers, in particular with high impact polystyrene. Efforts to compabilize these systems have also been reported. U.S. Pat. No. 4,748,203 discloses a polymer mixture of aromatic polycarbonate and rubber modified polystyrene. The agent to improve bonding is a polymer or copolymer of a vinyl aromatic with free carboxyl groups obtained by polymerization in the presence of an unsaturated carboxylic monomer (eg. maleic anhydride, acrylic/methacrylic acid or acrylate esters). U.S. Pat. No. 5,274,034 describes polymeric compositions comprising an aromatic polycarbonate, an aromatic polycarbonate containing acid or ester functionality, a styrene based polymer and a styrene polymer bearing oxazoline groups. As being useful in molding objects with matt surfaces. U.S. Pat. No. 5,204,394 illustrates mixtures comprising an aromatic polycarbonate, a copolymer containing styrene and a polymer grafted with polystyrene. U.S. Pat. No. 6,066,686 describes the use of epoxidized SBS copolymer as the compatibilizer and optionally polyesters such as PET, PBT or polyphenylene ether. U.S. Pat. No. 6,069,206 describes the use of a styrene-acrylonitrile copolymer with low acrylonitrile content and with a particular range of solubility parameter as a compatibilizer.
Compatibilizers for the blends of interest described in the prior art are based on copolymers where it is not possible to control microstructure (functionalized polymers are usually a random copolymer or melt functionalized polymer). The miscibility of such copolymers is compromised by its composition, limiting its application as a compatibilizer as in the case of random copolymers of styrene and maleic anhydride (Gan et. al., J. Appl. Polym. Sci., 54,317 (1994)).